This invention relates to a process for the selective noncatalytic reduction of the emission of pollutants from oil-fired boiler plants, with the metered feeding, in solutions miscible with the liquid fuel, of chemical compounds affecting the combustion step to the liquid fuel or to the combustion air; among these chemical compounds are salts of organic acids, such as naphthenates, octoates, tallates, salts of sulfonic acids, saturated or unsaturated fatty acids such as oleic acids, and tall oil, with metals selected from the group consisting of K, Ba, Mg, Ca, Ce, Mn, Fe, and rare earth metals; and organometallic compounds, such as carbonyl compounds, mixed cyclopentadienylcarbonyl compounds, or aromatic complexes of the transition metals Fe or Mn; especially ferrocene, manganocene or their derivatives; for example, ethylferrocene, and together with the metered introduction of a reducing agent into the combustion chamber.
As for the state of the art, attention is invited to an article by V. Hoenig and G. Baumbach, published in the collective volume from the VGB Conference Kraftwerk und Umwelt [Power Plant and the Environment], 1989, pp. 213-217, "Diminishing Pollutants in Heavy Oil Furnaces by Additives: Carbon Black, SO.sub.3, NOx"; the introduction to this article, cited hereinbelow, defines the greater technical area and the problem underlying the present invention.
The operators of heavy oil firing installations are likewise confronted by the problem of having to incorporate more stringent pollution control measures due to the critical pollutant emission values for furnace plants introduced in 1983 in the 13th BImSchV (Large-Scale Furnace Plant Regulation, GFAVO) and in 1986 in TA [Industrial Guidelines] "Air". Heavy fuel oil is presently combusted primarily in industry (e.g. steam and, respectively, hot-water generators, process heat) and to a minor extent still in peak load power plants. Most of the heavy oil firings lie within the power range of 50 MWt.sub.h (th = thermal), subject to TA "Air". Only a small number of oil-operated power plants is still in operation within the Federal Republic of Germany within the purview of GFAVO.
Although the total consumption of heavy fuel oil grade S has been reduced by more than one-half since the middle 1970's, the use of this oil is still of interest for some operators. Heavy oil consumption stabilized in the years 1986/87, primarily on account of low oil prices.
(Heavy fuel oil is obtained in crude oil processing and becomes a waste disposal problem if it is no longer possible to utilize heavy fuel oil grade S in a large number of so-called TA "Air" installations.)
Besides the nitric oxide emissions, the emissions of SO.sub.2, dust, and acidic flue gas components (SO.sub.3, sulfuric acid absorbed on fly dust or flue coke) represent a problem. The critical SO.sub.2 values of TA "Air" can be maintained by using an oil low in sulfur (maximally a content of 1%). In order to decrease NO.sub.x emission, these plants presently utilize predominantly primary measures of furnace technology and/or SNCR methods (selective noncatalytic reduction). One possibility of reducing the emissions of carbon black and acidic flue gas components resides in processing the oil with additives. For this purpose, special studies have been conducted at the Institute for Process Technology and Steam Boiler Operations of Stuttgart University (IVD).
U.S. Pat. No. 4,208,386 to Arand et al. discloses contacting combustion waste gases, having a residual oxygen content and a content of NO.sub.x, at an elevated temperature with urea, either as a solid or dissolved in a solvent, e.g. an alkanol of 1-3 carbon atoms, water, and a ketone of 3-4 carbon atoms. The urea is to be utilized in an amount effective for substantially reducing the NO.sub.x content in the waste gas. The temperatures to be employed are in a range of at least 1300.degree. F. in case an additional reducing agent is present, and at least 1600.degree. F. in the absence of a further reducing agent.
In another U.S. Pat. No. 4,325,924 to Arand et al., the disclosure relates to contacting NO.sub.x - containing combustion waste gases which, however, exhibit an excess of fuel with respect to the stoichiometric oxygen demand, with urea either as a solid or in solution at temperatures of above 1900.degree. F. and in quantities sufficient for a substantial decrease in the content of NO.sub.x. In this case, the equivalence ratio of fuel to oxygen has a value of larger than 1:1, especially larger than 1.05:1, and generally less than 1.5:1. The values thus obtained, as indicated in a table, confirm, in dependence on the urea concentration employed, significant reductions in NOx concentration; however, varying amounts of ammonia are likewise observed in the reduced waste gas, depending on the conditions of temperature and concentration utilized.
The use of ammonia for reducing the concentration of nitric oxide (NO) in combustion gases has been disclosed in U.S. Pat. No. 3,900,554. The object on which the patent is based resides in the selective reduction of NO in the presence of residual oxygen in the flue gas. The costs for a reducing agent in such a selective noncatalytic reduction of NO would correspond to the small amounts of NO to be reduced, rather than to the much larger quantities that would be needed for the reduction of NO as well as the residual oxygen.
Published International Patent Application WO 87/02025 concerns a method for reducing NO.sub.x in a waste gas stream from the combustion of carbonaceous fuels with oxygen excess, the formation of carbon-containing emissions being minimized. A dispersion of a dilute aqueous urea solution is injected into the waste gas stream at a temperature of above 2000 F, preferably above 2100.degree. F. The concentration of the urea solution and the droplet size in the dispersion are preferably at least 80% by weight solvent content and a droplet size in the range of 150-10,000 um. An aqueous solution of urea is injected into the said waste gas stream in the presence of an "oxygenated product", for example ethylene glycol, as a droplet dispersion at a temperature of above 1600` F.